The present invention is related to the fabrication of mask blanks for lithographic applications, particularly to fabricating mask blanks for Extreme Ultra-Violet Lithography (EUVL) and more particularly to a method for fabricating EUVL mask blanks composed of an Ultra-Low Expansion (ULE) substrate with a crystalline silicon surface.
EUVL is a leading candidate for the next generation of lithographic systems for fabrication of semiconductor microelectronics. EUVL technology development is progressing toward insertion into the production of integrated circuits with critical dimensions of 70 nm. The key difference between EUVL and conventional lithography is that the EUVL employs 13.4 nm light and therefore requires reflective optics that are coated with multilayers, typically alternating layers of molybdenum and silicon (Mo/Si). Deposition of low defect, uniform multilayer coatings on mask blanks is an area of intense development. Also, development of mask blanks or substrates that enable inspection for defects is another area of current development.
Thermal management of EUVL masks or substrates has become an important field in view of the current development efforts relating to EUVL systems. ULE materials, such as glass substrates, which have thermal coefficient advantages compared to silicon substrates, are being considered for the thermal management in EUVL mask blanks. Also, the vast experience with fabrication and processing, including the defect detection and classification results that have been obtained using crystalline silicon substrate materials, render crystalline silicon a desirable material for EUVL masks. Since mask blank defect inspection is one of the most important factors in determining mask blank yield and, as a consequence cost, the use of ULE mask blanks with a crystalline silicon surface is of great value to the EUVL program.
Various techniques for bonding silicon wafers to silicon or other materials are known in the art and include thermal-compression, anodic, etc. The bonding of silicon to a substrate can also be carried out at various temperatures including room temperature bonding. See S. N. Farren""s et al., Chemical Free Room Temperature Wafer to Wafer Direct Bonding, J. Electrochem. Soc. Vol. 142, No. 1, November 1995, 3949-3955.
The present invention provides a method that produces a mask blank which incorporates the thermal coefficient advantages of ULE substrates and the defect detection and classification advantages of crystalline silicon. The mask blanks composed of a ULE substrate with a crystalline silicon surface produced by the fabrication method of the present invention are applicable for EUVL systems now under development. Basically the fabrication method involves bonding a crystalline silicon wafer to a ULE wafer and thinning the crystalline silicon to a thickness of about 5-10 xcexcm.
It is an object of the present invention to provide an Extreme Ultra-Violet Lithography (EUVL) mask blanks using Ultra-Low Expansion (ULE) mask substrates.
A further object of the invention is to provide an EUVL mask blank wherein defect detection can be effectively carried out.
Another object of the invention is to provide ULE/silicon (ULE/Si) Extreme Ultra-Violet (EUV) mask blanks.
Another object of the invention is to provide a method for fabricating mask blanks composed of an ULE substrate with a crystalline silicon surface.
Another object of the invention is to provide a mask blank which combines the advantages of ultra-low thermal expansion of selected materials such as glass, plastics, and ceramics with the defect inspection advantages of crystalline silicon.
Another object of the invention is to provide a method for fabricating EUVL mask blanks by bonding a crystalline silicon wafer to an ULE wafer and thinning the silicon wafer to a thickness of about 5-10 xcexcm crystalline silicon film.
Other objects and advantages of the present invention will become apparent from the following description and accompanying drawings. The invention involves a method for the fabrication of ultra-low expansion/silicon extreme ultraviolet mask blanks. The ULE substrate enables the necessary thermal management in EUVL mask blanks while the silicon surface film on the substrate enables defect inspection, which is one of the most important factors in determining mask blank yield and as a consequence, cost, of the mask blank. By combining ULE/Si into a mask blank, there is produced an effective EUV mask blank on which reflective multilayers may be deposited for EUVL applications. Basically the method involves bonding a crystalline silicon wafer to an ultra-low expansion wafer such as selected glass, plastic or ceramic and then thinning the silicon to a desired thickness (e.g., 5-10 xcexcm). The silicon can be thinned and polished by chemical and mechanical polishing and thermal oxidation followed by etching, where needed, to produce a crystalline silicon surface on the ULE substrate.